RE series superconductors (RE: rare earth element) is conventionally known as a kind of high temperature superconductors, which show superconductivity at a liquid nitrogen temperature (77 K) or more. In particular, an yttrium series superconductor expressed by a chemical formula YBa2Cu3O7-y is typical (hereinafter referred to as a Y series superconductor or a YBCO).
A superconducting wire using an RE series superconductor (hereinafter referred to as an RE series superconducting wire) generally has a laminated structure in which an intermediate layer, a layer of an RE series superconductor (hereinafter referred to as an RE series superconducting layer), and a stabilization layer are formed on a tape-shaped metal substrate in this order.
The RE series superconducting wire is, for example, manufactured as follows: a film of a biaxially oriented intermediate layer is formed on a low magnetic non-oriented metal substrate (for example, HASTELLOY (registered trademark)), and a film of an RE series superconducting layer is formed on the biaxially oriented intermediate layer by Pulsed Laser Deposition (PLD), Metal Organic Chemical Vapor Deposition (MOCVD), or the like.
It is known that the voltage-current characteristics of such a high temperature superconducting wire greatly depends on the crystal orientation, especially the biaxial orientation, of the superconductor. Accordingly, it is necessary to improve the crystallinity of an intermediate layer, which is used as a foundation, in order to obtain a superconducting layer having a high biaxial orientation. As one of the methods, Ion Beam Assisted Deposition (IBAD) is disclosed in Patent Documents 1 and 2. The IBAD method is a method of film formation by depositing particles evaporated from an evaporation source while radiating an assist ion beam to a deposition aspect from an oblique direction.
Patent Document 1 lists yttrium-stabilized zirconia (YSZ), magnesium oxide (MgO), and strontium titanate (SrTiO3) as the evaporation sources applicable to the IBAD method. Moreover, Patent Document 2 cites a fluorite type material (CeO2, YSZ, and the like), a pyrochlore type material (GZO(Gd2Zr2O7) and the like), a rare earth C type material (Y2O3 and the like), a rock salt type material (MgO and the like), ReO3 type material (WO3 and the like), and perovskite type material (LaAlO3 and the like) as the evaporation sources applicable to the IBAD method. In particular, a thin film made of MgO, which is a rock salt type material, enables the obtainment of a high biaxial orientation, and the thin film made of MgO is accordingly the main current of development.
The conventional superconducting wire includes a diffusion preventing layer (hereinafter referred to as a barrier layer) on the metal substrate thereof lest the diffusion of cations (Ni, Mo, Mn, and the like) from the metal substrate should reach the superconducting layer, and furthermore, a reaction suppressing layer (hereinafter referred to as a bed layer) suppressing the reactions of the diffusion preventing layer and MgO. Moreover, a cap layer, which is made of CeO2 or the like, is formed on the biaxially oriented layer of the conventional superconducting wire in order to protect the biaxially oriented layer, made of MgO, which easily reacts with the air, and to enhance the lattice matching with the superconducting layer (for example, a YBCO).
Consequently, as shown in FIG. 4, the intermediate layer 50 of the conventional superconducting wire is composed of the barrier layer 51, the bed layer 52, the biaxially oriented layers 53 and 54, and the cap layer 55. Incidentally, in FIG. 4, the biaxially oriented layer 53 is an MgO layer formed as a film by the IBAD method, and the biaxially oriented layer 54 is an MgO layer formed as a film on the IBAD-MgO layer 53 by the PLD method or the like.
In the following, a long tape-shaped base composed of a metal substrate and an intermediate layer will be called a tape-shaped base for a superconducting wire. A film of a superconducting layer made of the YBCO or the like is formed on the tape-shaped base for a superconducting wire (on the cap layer 55 in FIG. 4).